Malignant tachyarrhythmia, for example, ventricular fibrillation, is an uncoordinated contraction of the cardiac muscle of the ventricles in the heart, and is the most commonly identified arrhythmia in cardiac arrest patients. If this arrhythmia continues for more than a few seconds, it may result in cardiogenic shock and cessation of effective blood circulation. As a consequence, sudden cardiac death (SCD) may result in a matter of minutes.
In patients with a high risk of ventricular fibrillation, the use of an implantable cardioverter defibrillator (ICD) system has been shown to be beneficial at preventing SCD. An ICD system includes an ICD that is a battery powered electrical shock device, that may include an electrical housing electrode (sometimes referred to as a can electrode), that is typically coupled to one or more electrical lead wires placed within the heart. If an arrhythmia is sensed, the ICD may send a pulse via the electrical lead wires to shock the heart and restore its normal rhythm. Owing to the inherent surgical risks in attaching and replacing electrical leads directly within or on the heart, extracardiovascular ICD systems have been devised to provide shocks to the heart without placing electrical lead wires within the heart or attaching electrical wires directly to the heart.
Some tachyarrhythmias may be terminated by anti-tachycardia pacing (ATP) therapy. ICDs have been configured to attempt to terminate some detected tachyarrhythmias by delivery of ATP prior to delivery of a shock. Additionally, ICDs have been configured to deliver relatively high magnitude post-shock pacing after successful termination of a tachyarrhythmia with a shock, in order to support the heart as it recovers from the shock. Some ICDs also deliver bradycardia pacing, cardiac resynchronization therapy (CRT), or other forms of pacing.
To capture the heart, an extracardiovascular ICD would likely need to deliver pacing pulses having a higher magnitude than those delivered by a conventional transvenous ICD coupled to intracardiac leads due to the greater distance between the electrodes of the extracardiovascular ICD and the heart. Higher magnitude pacing pulses delivered by an extracardiovascular ICD may capture other patient tissue in addition to the heart and/or cause patient discomfort. Consequently, it has been proposed to concomitantly implant a separate cardiac pacing device with an extracardiovascular ICD configured to deliver anti-tachyarrhythmia shocks. The cardiac pacing device may be an intracardiac pacing device (IPD) configured to be wholly implanted within the heart.